It is known to employ a laser source to erode surfaces of workpieces and the like. Such apparatus is in general relatively complex and demands highly skilled use. It is an object of the present invention to provide improved and simplified apparatus and method for eroding surfaces.
It is also an object of the present invention to provide an improvement whereby laser techniques can be applied to sensitive surfaces and in particular to objects in which it would be undesirable to affect underlying layers.
In the field of medicine, a known technique for the treatment of certain forms of myopia is surgically to remove a segment of the collagen sub-surface layer of the eye, to reshape the removed segment as by surgical grinding, and to restore the reshaped segment in the eye. The eye heals by reformation of the outer cellular layer over the reshaped collagen layer. Alternatively, a layer of the cornea is opened up as a flap, an artificial or donor lenticular implant is inserted under the flap, and the flap is sutured up again.
It is a further object of this invention to provide an improved and less traumatic method and apparatus for reshaping the cornea of the eye.
Various other surgical techniques for reprofiling of the corneal surface have also been proposed. One increasingly common technique is radial keratotomy, in which a set of radial incisions, i.e. resembling the spokes of a wheel, are made in the eye to remedy refractive errors such as myopia (nearsightedness). As the incisions heal, the curvature of the eye is flattened, thereby increasing the ocular focal distance. The operation is not particularly suitable for correction of hyperopia (farsightedness) and can pose problems if the surgical incisions are uneven or too deep.
The use of a laser beam as a surgical tool for cutting incisions, a so-called "laser scalpel", has been known for some time (see for example U.S. Pat. No. 3,769,963 to Goldman et al). In 1980, a study was made of the damage which might be inflicted on the corneal epithelium by exposure to the recently developed excimer laser (see Taboada et al, "Response of the Corneal Epithelium to ArF excimer laser pulses" Health Physics 1981, Volume 40, pp 677-683). At that period, surgical operations on the cornea were commonly carried out using diamond or steel knives or razor, and further such techniques were still being studied (see for example Binder et al, "Refractive Keratoplasty" Arch, Ophthalmol. May 1982, Vol. 100, p 802). The use of a physical cutting tool in corneal operations, and the insertion of an implant under a flap, continue to be widely practised and techniques further developed up to the present day (see for example "Refractive Keratooplasty improves with Polysulfone, Pocket Incision" Ophthalmology Times, July 1, 1986).
It has been suggested in European Patent Application No. 01518699 of L'Esperance, to perform controlled ablative photodecomposition of one or more selected regions of a cornea using a scanning action on the cornea with a beam from an excimer laser. Because of the scanning action, it is necessary for L'Esperance to bring his laser beam to a small spot, typically a rounded-square dot of size 0.5 mm by 0.5 mm.
L'Esperance suggests that myopic and hyperopic conditions can be reduced by altering the curvature of the outer surface of the cornea by repeatedly scanning the cornea with an excimer laser beam having this standard small spot size by varying the field which is scanned during successive scans, so that some areas of the cornea are scanned more often than others. In this way, it is claimed that the surface can be eroded by different amounts depending on the number of times they are scanned by the spot. Additionally, he suggests that certain severe myopic and hyperopic conditions may be treated with a reduced removal of tissue by providing the outer surface of the cornea with a new shape having Fresnel-type steps between areas of the desired curvature.
In practice, complex apparatus is required to cause a laser beam to scan with the precision required if the eroded surface is to be smooth. Thus, if successive sweeps of a scan overlap, there will be excessive erosion in the overlap area, whereas if they fail to meet, a ridge will be left between the sweeps. The compression of the excimer laser beam to a small spot will increase the beam energy density, which will tend to exacerbate these problems. It is not clear that L'Esperance has found a suitable scanning system, since in one embodiment he attempts to control the laser beam by a magnetic field.
Additionally, the scanning method is inherently time-consuming even with highly refined techniques and apparatus, since the laser beam is only eroding a very small part of the total area to be treated at any given moment. Furthermore, such a scanning system can cause rippling effects on relatively soft materials such as corneal tissue.
It is therefore a further object of the present invention to provide a method and apparatus for eroding a surface using a laser which does not require scanning of the area of the surface to be eroded.
Another technique for corneal reshaping, described in British Patent Application No. 8604405 and herein incorporated by reference, involves the use of a laser photoablation apparatus in which the size of the area on the surface to which the pulses of laser energy are applied, is varied to control the reprofiling operation. In one preferred embodiment, a beam-shaping stop or window is moved axially along the beam to increase or decrease the region of cornea on which the laser radiation is incident. By progressively varying the size of the exposed region, a desired photoablation profile is established in the surface. For further details on this technique, see also, Marshall et al, "Photo-Ablative Reprofiling of the Cornea Using an Excimer Laser: Photorefractive Keratoctomy", Vol. 1, Lasers in Ophthalmology, pp 21-48 (1986).
Although this technique for varying the size of the exposed region is a substantial improvement over physical shaping (i.e. scalpel) techniques and laser spot scanning protocols, a considerable number of optical elements and control systems still are required for precise operation, particularly on human corneal tissue. There exists a need for better and simpler procedures for shaping surfaces, particularly the surfaces of biological tissues, such a corneal tissue.